A conventional tomographic imaging apparatus which obtains a tomographic image using an ultrasonic wave includes: a probe for transmitting an ultrasonic wave to a sample and receiving the reflected ultrasonic wave; a transmitting portion for supplying an ultrasonic signal to the probe; a receiving portion for receiving the reflected wave; and a unit for converting the received reflected wave signal into a luminance signal for visualization. When a time-series tomographic image acquired by the apparatus is used, it is possible to observe an inside of a sample. In the apparatus according to one mode, a unit for performing scanning of a probe two-dimensionally scans a sample with an ultrasonic wave to obtain a three-dimensional image.
Meanwhile, in examining an inspection object, apparatuses which display not only a morphologic image but also a functional image have progressively been developed in recent years. As one of such apparatuses as described above, there is an apparatus which utilizes a photoacoustic spectroscopy. In the photoacoustic spectroscopy, visible light, near-infrared light, or mid-infrared light each having a predetermined wavelength is applied to the inspection object. Then, a specific substance inside the inspection object absorbs energy of the applied light, and as a result of the absorption, an elastic wave (photoacoustic wave) is generated and detected. In this manner, concentration of the specific substance is quantitatively measured. The specific substance inside the inspection object is, for example, glucose or hemoglobin contained in blood. A technology of acquiring a photoacoustic image through the photoacoustic spectroscopy is disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-507952. The technology is referred to as a photoacoustic tomography (PAT).
In addition, Japanese Patent Application Laid-Open No. 2005-021380 discloses a method of reconstructing both a photoacoustic image and a normal ultrasonic echo image using a one-dimensionally-arranged electromechanical transducer which is common to the both images; and a structure in which a lighting system using a glass fiber is provided between one-dimensionally-arranged electromechanical transducers. According to Japanese Patent Application Laid-Open No. 2005-021380, the ultrasonic echo image and the photoacoustic image are simultaneously acquired, thereby displaying a morphologic image and a functional image. In this case, a common probe is used to transmit and receive an ultrasonic wave for forming the ultrasonic echo image and to receive a photoacoustic wave for forming the photoacoustic image.
It should be noted herein that an elastic wave generated by the photoacoustic spectroscopy (photoacoustic imaging method) is referred to as a photoacoustic wave, and a sonic wave which is transmitted and received in a normal pulse echo method is referred to as an ultrasonic wave.
The frequency band of a photoacoustic wave used in the photoacoustic spectroscopy is generally lower than the frequency band of an ultrasonic wave used in ultrasonography. For example, the frequency band of the photoacoustic wave is distributed within a range of 200 KHz to 2 MHz with 1 MHz being a center frequency. The distribution of the frequency band of the photoacoustic wave is lower than a center frequency of 3.5 MHz of the ultrasonic wave used in ultrasonography. According to Japanese Patent Application Laid-Open No. 2005-021380, the common probe is used to receive the photoacoustic wave and the ultrasonic wave used in ultrasonography.
However, as described in Japanese Patent Application Laid-Open No. 2005-021380, when the common probe is used to receive the photoacoustic wave and the ultrasonic wave which have frequency bands different from each other, there arises a problem that spatial resolution is deteriorated in the ultrasonic image. In order to solve the above-mentioned problem, a harmonic imaging method is used in Japanese Patent Application Laid-Open No. 2005-021380. However, a signal contained in a harmonic component is attenuated more than a signal contained in a fundamental component, and hence there is a fear that sensitivity may be decreased.
In a case where the frequency band of the photoacoustic wave and the frequency band of the ultrasonic wave are remarkably separated from each other (for example, the center frequency band of the photoacoustic wave is approximately 1 MHz and the center frequency band of the ultrasonic wave is approximately 10 MHz), the above-mentioned problem becomes more remarkable when the common probe is used to receive the waves as described in Japanese Patent Application Laid-Open No. 2005-021380.
Moreover, with regard to a photoacoustic-ultrasonic system including an ultrasonic probe and an optical system, the following problem arises. Specifically, according to Japanese Patent Application Laid-Open No. 2005-021380, for generation of a photoacoustic wave, a laser light is introduced using an optical fiber. However, in order to generate the photoacoustic wave, an extremely strong laser light is required, which may adversely affect the fiber. Particularly, when a sample is thick so that light is attenuated to a large degree, the above-mentioned problem becomes more serious.
According to an experiment conducted by the inventors of the present invention using a dummy inspection object, it was found that, in order to generate a photoacoustic wave strong enough to detect a sample having a thickness exceeding the order of “cm”, the intensity of the laser light introduced into the optical fiber exceeds several tens kJ/cm2, which increases a burden on the optical fiber. Therefore, the optical fiber may not be selected as the optical system for detecting a sample having a certain volume.